Add the ability to run pre/post commands with task data

This is useful for logging runner task status to external sources. The hooks themselves
execute arbitrary commands, passing in a json blob with task stats as an argument.
Update readme to reflect runner config changes.
Fix readme formatting.
Add tests for pre/post hooks
Make tests more readable and gurantee that logfile will be deleted via with_setup.
Add message to failure of post_hook specific assertion.
Merge pull request #10 from mrrrgn/prepost

Prepost